Circuit for limiting the output voltage of a power transistor

Abstract: A circuit for limiting the output voltage (Vc) from a power transistor (T1) connected in series with a resonant load (ZL) between a first pole (Vs) of a supply voltage generator and a second pole of the generator (GND), comprises a semiconductor junction element (40), in particular a diode of the SCR type, having an anode terminal (A) connected to the first pole (Vs) of the supply voltage generator, a cathode terminal (K) connected to a common circuit node (Vc) between the power transistor (T1) and the resonant load (ZL), and a control terminal (G) connected to a reference voltage of predetermined value (E).

Description: Field of the Invention This invention relates to a circuit for limiting the output voltage from a power transistor employed to drive a resonant electric load which is connected to a supply pole. The invention concerns in particular, but not exclusively, a power device used in the automotive industry for driving an ignition coil or a transformer. For this purpose, either a single transistor of the bipolar type, or a pair of transistors in Darlington configuration, or a MOS type of transistor are used as the power element. The power transistor is periodically turned on and off by a driver circuit as controlled by an appropriate input signal. In essence, the power transistor acts as a switch which is open or closed by the input signal supplied to the driver circuit. With the switch in the closed state, a current flows through the load. As the switch is opened, a positive overvoltage establishes between the load and the switch which is followed by a series of negative voltage peaks dependent on the parasitic capacitances of the load and the way the energy which has been stored in the load is discharged upon the switch being opened. These negative voltage peaks pull the output voltage of the device even several volts below ground level. Where the driver circuit and power device are provided in the integrated circuit form, certain parasitic components associate with the active components of the driver circuit which, as said voltage drops below ground level, enter a conduction state, thereby shorting out the active components of the driver circuit and impairing the operation of the latter. Background Art In order to prevent the power device output from attaining a negative voltage value relative to ground, thereby triggering on the parasitic components, a circuit must be connected to that output which can provide real time control of the output voltage to hold it within predetermined limits. A known technical approach to providing such a limiter circuit is disclosed in European Patent Application No. 90202775.4, published under No. 0425024 on May 5, 1991. The limiter circuit described in the above patent application includes a comparator which performs a comparison of the output voltage from the power device with a predetermined reference voltage. Where the output voltage is slightly below the reference voltage, the comparator supplies the load with a current effective to prevent the output voltage from dropping below the reference voltage. This prior approach, while being advantageous from many aspects, is a structurally complex one and uses up an area of the integrated circuit which accounts for a significant proportion of the total area of the device. Its structural complexity unavoidably implies, moreover, such added disadvantages as increased cost and rate of device rejects. In the United States Patent no. 3,662,184 published on 9 May 1972 is disclosed a circuit arrangement for sensing and limiting voltages induced in a primary winding of a transformer from a secondary winding. This circuit arrangement includes a diode and a switching transistor means comprising a first and a second transistor connected essentially as a PNPN device. Voltages induced, in primary winding which tend to reverse bias the diode appears as a potential difference across diode and this voltage is applied to the emitter-base junction of the first transistor rendering both transistors heavily conductive. The conductive condition of the switching transistor means is a very low impedance on primary winding which is reflected to secondary winding. The underlying technical problem of this invention is to provide a limiter circuit as indicated, which has such structural and functional features as to prevent the output of the power device, that is its collector connected to the load, from attaining a negative voltage relative to ground and triggering on the parasitic components. This technical problem is solved by a circuit for limiting the output voltage from a power transistor, which comprises a semiconductor junction element, specifically a silicon controlled rectifier, as defined in Claim 1. The features of this invention will be more clearly apparent from the following detailed description of embodiments thereof, shown by way of example and not of limitation in the accompanying drawings, in which: Figure 1 is a diagram showing schematically a driver circuit and a power device with a load driven thereby; Figure 2 illustrates the pattern vs. time of the voltage present at the output node of the power transistor; Figure 3 is an enlarged vertical section view showing schematically a portion of an integrated circuit incorporating a prior art driver circuit and power transistor arrangement; Figure 4 is a wiring diagram of the power transistor and associated limiter circuit according to the invention; Figure 5 shows a modified embodiment of the limiter circuit in Figure 4; Figure 6 shows the patterns vs, time of certain electric signals present in the circuits of the preceding Figures; and Figure 7 is an enlarged vertical section showing schematically a portion of an integrated circuit incorporating the power transistor and limiter circuit of this invention. Detailed Description With reference in particular to the example of Figure 1, an electric load ZL, schematically represented by its equivalent electric circuit R-L-C, is connected to a circuit node at a voltage Vs. The load ZL is driven by a power device, specifically a bipolar transistor T1 of the NPN type, which is turned on and off by a signal from a driver circuit 1, known per se, under control by an appropriate input signal Vin. The output voltage of the power device is designated Vc. Shown in Figure 2 is the pattern vs. time of the voltage Vc when the transistor Tl is in the off state. It can be seen that a positive voltage first establishes itself at said node, followed by a series of negative voltage peaks. These negative voltage peaks will pull the output voltage of the device even several volts below ground level. With reference to Figure 3, shown in vertical section therein is a portion of the integrated structure of the circuit illustrated by Figure 1. This portion comprises an N+ doped substrate, indicated as region 2, wherein a power transistor T1 is formed whose collector, base and emitter regions are respectively defined by regions 3, 7 and 8. Formed in the region 2 as a part of the driver circuit 1, is a second transistor T4 whose collector, base and emitter regions are respectively defined by regions 4, 9 and 6. The transistor T4 is contained in an isolation well formed of a buried region 5 of the P type and two regions 10, 11 of the P+ type. The integrated structure just described implies the presence of two parasitic transistors T5 and T6. The base of the transistor T5 is in common with the collector of the transistor T6, formed in the region 4 of the N+ type, its emitter is formed in the region 9 of the P type, and its collector is in common with the base of the transistor T6, in the region 5 of the P type. Lastly, the emitter of the transistor T6 is formed within the region 3 of the N type. The connection pattern of the parasitic transistors T5, T6 is illustrated in Figure 3 by a wiring diagram superimposed on the cross-sectional representation of the integrated structure. The principle of this invention will now be explained with reference to the embodiment shown in Figure 4. The power device T1 consists of a bipolar transistor of the NPN type. The emitter of the transistor T1 is connected to ground, whilst its emitter is connected to a load ZL, specifically a coil or a transformer. The other end of the load ZL is connected to a node at a voltage Vs. Associated with the collector of the transistor T1 is a limiter circuit 40 for its related output voltage Vc. The limiter circuit 40 includes a semiconductor junction element, in particular a silicon controlled rectifier, or SCR, having an anode terminal A, a cathode terminal K, and a control terminal G. The anode terminal A is connected to the node at the voltage Vs, the cathode terminal K is connected to the common circuit node between the power transistor T1 and the resonant load ZL, and the control terminal G is connected to the positive pole of a generator E of a predetermined value. The negative pole of this generator is connected to the circuit ground. In the embodiment shown in Figure 5, the silicon controlled rectifier is formed of two transistors T2, T3 of a mutually complementary type. In particular, these transistors T2, T3 are bipolar transistors of the PNP and NPN type, respectively. The emitter of the first transistor T2 is the anode terminal A of the SCR, whereas the emitter of the second transistor T3 is the cathode terminal K of the diode. The collector of the first transistor T2 is connected to the base of the second transistor T3, and is the control terminal G of the SCR, and the base of the first transistor T2 is connected to the collector of the second transistor T3. Also shown in Figure 5 is an embodiment circuit-wise of the fixed voltage generator E. This reference voltage is obtained through a diode D1 which is forward biased by a resistor R1. A terminal of this resistor is connected between the circuit node at the voltage Vs and the anode terminal of the diode D1, the cathode terminal of the diode D1 being connected to the circuit ground. Shown in Figure 6 is the pattern vs. time of certain electric quantities which appear in the circuits described in the foregoing and illustrated by Figures 1, 4 and 5. In particular, the curve designated 51 represents the pattern vs. time of the voltage Vc present at the output node of the power device as the transistor T1 is turned off and in the absence of a limiter circuit for said output voltage. It can be seen that a positive overvoltage first establishes itself at said node which is followed by a series of negative voltage peaks. The curve designated 52 represents the pattern vs. time of the voltage Vc present at the output node of the power device as the transistor T1 is turned off, in the presence of the limiter circuit of this invention. It can be seen that the value of the voltage Vc remains positive all the time. The curve designated 53 illustrates the pattern vs. time of the current Is through the SCR 40. The controlled rectifier 40 will be conducting when the voltage between the control terminal G and the cathode terminal exceeds a given threshold value, typically in the 0.6 to 0.7 volts range. Shown in Figure 7 is a vertical section view of the integrated structure of the limiter circuit depicted schematically in Figure 5, and of that of the power transistor T1. This structure comprises an N+ doped substrate, denoted by 2, wherein the power transistor T1, the two transistors T2 and T3, the resistor R1 and the diode D1 are formed. The collector, base and emitter regions of the power transistor T1 are respectively defined by the regions 3, 7 and 8. The two transistors T2, T3 of the limiter circuit in Figure 5 are formed in four superimposed layers which are doped with impurities of the P type and the N type, alternately. More specifically, the base of the transistor T2 is in common with the collector of the transistor T3 in a region 13 of the N+ type; its emitter is formed in a region 14 of the P type; its collector is in common with the base of the transistor T3 in a region 12 of the P type; and the emitter of the transistor T3 is formed within the region 3 of the N type. It will be appreciated that the structure of the regions forming such transistors substantially corresponds to the structure of the parasitic transistors T5, T6 present in the driver circuit portion shown in Figure 3. This similarity of the two structures enhances the simplicity of the solution proposed, because it makes for extremely simple integration of the limiter circuit to the integrated circuit to be protected. Formed in the region 2, again within the structure of Figure 7, are also the resistor R1, defined in a region 15 of the P type, and the diode D1, having its anode formed in a region 16 of the P type and cathode formed in a region 17 of the N+ type. The wiring diagram of the integrated components just described is shown, again in Figure 7, superimposed on the sectional view of the integrated structure. As to the operation of the circuit of Figure 4, the silicon controlled rectifier, contained in the limiter circuit 40, will be conducting current as the voltage between the control terminal G and the cathode terminal K exceeds a given threshold value, typically in the 0.6 to 0.7 volts range. Since the voltage at the control terminal G of the SCR is held constant by the voltage generator E, the diode will be conducting as the voltage at its cathode terminal K reaches a value close to zero volts. The current supplied to the load by the SCR maintains the voltage Vc at the collector of the power transistor T1 positive, as shown by the curve 52 in Figure 6. In the embodiment shown in Figure 5, the limiter circuit 40 comprises two transistors T2, T3, respectively of the PNP and NPN type, and the voltage reference E is formed by a diode D1 forward biased by a resistor R1. The base of the transistor T3, which represents the control terminal of the SCR, is connected to the anode of the diode D1, whereby, when the voltage Vc tends to go negative, the transistor T3 begins conducting through the resistor R1; as a result, the transistor T2 will also be conducting and supply the resonant load with the required current to hold the collector of the transistor T1 at a voltage close to zero during the time when it would tend to go negative due to the capacitance present at the resonant load. Again in Figure 5, it can be seen that, as Vc becomes positive, perhaps by more than 400 volts in the instance of a circuit driving an automotive ignition coil, the emitter-base junction of the transistor T3 is also to withstand that voltage. In fact, its base is connected to a potential of about 0.7 volts, whilst its emitter is connected to the collector of the high-voltage transistor T1. This junction, which is to withstand high voltages, can be readily implemented in the VIPower technology employed for making the integrated structures shown in Figures 3 and 7. Figure 7 shows indeed an embodiment wherein the base-emitter junction of the transistor T3, the regions 3 and 12, is provided by diffused regions similar to those, the regions 3 and 7, utilized to form the collector and base, respectively, of the transistor T1, which is a high-voltage power transistor. To summarize, this invention does provide a circuit for limiting the output voltage from a power transistor arranged to drive a resonant load, which circuit is quite simple construction-wise and affords significant savings in integrated circuit area compared to prior art solutions. In particular, the area occupied by the limiter circuit of this invention is ten times smaller than the area occuped by the prior circuit described in the above-referenced patent application EP-A-0 425 024. Furthermore, the manufacture of the circuit is specially advantageous where the circuit is implemented in the VIPower technology, as is normally used for power devices which incorporate a logic control portion.

Claims: A circuit for limiting the output voltage (Vc) from a power transistor (T1) connected in series with a resonant load (ZL) between a first pole (Vs) of a supply voltage generator and a second pole of said generator (GND), comprising a semiconductor junction element (40) having a first terminal (A), connected to said first pole (Vs), a second terminal (K) connected to a common circuit node (Vc) between the power transistor (T1) and the resonant load (ZL) and a third terminal (G), said semiconductor junction element (40) being provided by first (T2) and second (T3) bipolar transistors of a mutually complementary type each having an emitter, a collector and a base, wherein the emitter of the first transistor forms the first terminal (A) of the semiconductor junction element (40), the base of the first transistor connects to the collector of the second transistor, the collector of the first transistor connects to the base of the second transistor to form the third terminal (G) of the semiconductor junction element (40) and the emitter of the second transistor forms the second terminal (K) of the semiconductor junction element (40), characterised in that the third terminal (G) of the semiconductor junction element (40) is connected to a voltage reference (E) of a predetermined value. A circuit according to Claim 1 characterised in that the semiconductor junction element (40) is a silicon controlled rectifier, or SCR, and said first (A), second (K) and third (G) terminals correspond, respectively, to the anode, cathode and control terminals. A circuit according to Claim 2, characterised in that said voltage reference (E) is obtained through a diode (D1), having an anode and a cathode terminal, forward biased by a resistor (R1). A circuit according to Claim 3, characterised in that said resistor (R1) is connected between the first pole (Vs) of the supply voltage generator and the anode terminal of the diode (D1), and the cathode terminal of the diode (D1) is connected to the second pole (GND) of the supply voltage generator. A circuit according to any of Claims 1 to 4, characterised in that the semiconductor junction element (40) is formed on a semiconductor substrate, along with said power transistor (T1), in four superimposed layers (3,12,13,14) alternately doped with impurities of the N type and the P type. A circuit according to Claim 5, characterised in that the cathode terminal (K) of the semiconductor junction element (40) is formed in the first (3) of said layers, doped with impurities of the N type; the control terminal (G) of the semiconductor junction element (40), corresponding to the base region of the second transistor (T3) and to the collector region of the first transistor (T2), is formed in the second (12) of said layers, doped with impurities of the P type; the anode terminal (A) of the semiconductor junction element (40), corresponding to the emitter region of the first transistor (T2), is formed in the fourth (14) of said layers, doped with impurities of the P type; and the collector region of the second transistor (T3) and the base region of the first transistor (T2) are formed in the third (13) of said layers, doped with impurities of the N type.

Inventors: PALARA SERGIO, PALARA, SERGIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H03K17
IPC codes: H03K17, H01L27, H01L21, H01L29